1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA ABP 9-2015Hazards of Working on Energized Electrical EquipmentA NEMA Low Voltage Distribution Equipment Section Document ABP 9-2015 Hazards of Working on Energized Electrical Equipment Published by National Electrical Manufa
2、cturers Association 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2015 National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Literar
3、y and Artistic Works, and the International and Pan American copyright conventions. 2015 National Electrical Manufacturers Association NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the
4、 document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. NEMA standards and guideline publications, of which the document contained herein is one, are developed through a volunt
5、ary consensus standards development process. This process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA administers the process and establishes rules to promote fairness in the development of consensus, it d
6、oes not write the document and it does not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of
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8、information published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products or services by virtue of this standar
9、d or guide. In publishing and making this document available, NEMA is not undertaking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to perform any duty owed by any person or entity to someone else. Anyone using this document should rely on
10、 his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be available from other sources, which the user
11、 may wish to consult for additional views or information not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this document. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purp
12、oses. Any certification or other statement of compliance with any health- or safety-related information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. NEMA ABP 9-2015 Page i 2015 National Electrical Manufacturers Associ
13、ation Foreword This is an update to “Hazards of Working Electrical Equipment Hot,” a NEMA white paper originally published in 1994. To ensure that a meaningful publication was being developed, draft copies were sent to a number of groups within NEMA having an interest in this topic. Their resulting
14、comments and suggestions provided vital input prior to final NEMA approval and resulted in a number of substantive changes in this publication. This publication will be reviewed periodically by the Molded Case Circuit Breaker Product Group of the Low Voltage Distribution Equipment Section of NEMA fo
15、r any revisions necessary to keep it up to date with evolving technology. Proposed or recommended revisions should be submitted to: Vice President, Technical Services National Electrical Manufacturers Association 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 This white paper was develope
16、d by the Molded Case Circuit Breaker Product Group of the Low Voltage Distribution Equipment Section of NEMA. Approval does not necessarily imply that all members of the Product Group voted for its approval or participated in its development. The Molded Case Circuit Breaker Product Group had the fol
17、lowing members at the time it was approved: ABB Inc.Wichita Falls, TX Eaton CorporationPittsburgh, PA General ElectricPlainville, CT Siemens Industry, Inc.Norcross, GA Schneider Electric USAPalatine, IL NEMA ABP 9-2015 Page ii 2015 National Electrical Manufacturers Association NEMA ABP 9-2015 Page 1
18、 2015 National Electrical Manufacturers Association Working on energized electrical equipment is a significant safety concern of the electrical industry. Accident reports continue to confirm that people responsible for installation or maintenance of electrical equipment often do not turn off the pow
19、er source before working on that equipment. NFPA 70E, Standard for Electrical Safety in the Workplace, recognizes that there are circumstances that allow working on live, energized electrical equipment. As a result, recommendations and requirements are defined for levels of protective systems and ap
20、paratus that must be in place when personnel are working near live, energized electrical equipment. NFPA 70E also outlines strict guidelines for the various levels of Personnel Protective Equipment (PPE) that must be worn by personnel in certain hazardous situations. NOTENFPA 70E defines “working on
21、” (energized electrical conductors or circuit parts) as intentionally coming into contact with energized electrical conductors or circuit parts with the hands, feet, or other body parts, with tools, probes, or test equipment, regardless of the PPE a person is wearing. The purpose of this article is
22、to alert electrical contractors, electricians, facility owners and managers, and other interested parties to some of the hazards of working on energized equipment and to emphasize the importance of turning off the power before working on electrical circuits. I. Why Should the Power Be Turned Off? Da
23、nger of Arcing Faults. An arcing fault occurs when conductors of opposite polarity are accidentally bridged by a conductive object or bridged to grounded metal. Metal screwdrivers, wrenches, fish tapes, and test instruments, for example, have all been found to have made inadvertent contact while per
24、sons were working on live equipment. This can cause an arcing fault that is limited only by the total impedance of the circuit. The arcing will continue until a circuit breaker, fuse, or equipment ground fault protection device on the line side of the fault opens the circuit. Even if the overcurrent
25、 protective device opens the circuit without any intentional delay, portions of the conductors and other metallic materials in the path of the arc might explode violently, creating plasma and showering the area with hot molten metal that can cause severe burns or death. The flash associated with the
26、 arc can cause permanent damage to eyes and ears. The energy blast wave from the arc flash can cause external and internal injuries. The pressure wave from an energy blast might expel shrapnel toward the worker, penetrating clothing and/or the body, and might even throw a worker across the room. Dan
27、ger of Normal or Abnormal Switching Operations. Many components of an electrical system (e.g., switches, circuit breakers, and contactors) are required to be mounted in an enclosure intended to prevent accidental contact with live electrical parts. Enclosures are also intended to contain byproducts
28、from normal or abnormal switching operations. When a switch, circuit breaker, or contactor opens a circuit carrying rated current, or perhaps an overcurrent, an arc is established across the contacts of the device. Under pressure, hot gasses and tiny metal particles might be expelled from the device
29、. This is a normal consequence, and the closed enclosure contains the hot gasses and particles, protecting personnel from possible severe injury. However, if the enclosure cover is opened or removed while the equipment is still energized, and a switching operation occurs, severe burns to the body ca
30、n result from the hot gases and ejected metal particles; eye damage can occur as a result of the associated flash. Enclosures for electrical equipment are designed to safely contain normal and abnormal conditions. They cannot perform as intended if opened when equipment is energized. Danger of Shock
31、 or Electrocution. The human body conducts electrical current! A circuit path can be created through both arms, through an arm or a leg to ground, or through any contact of the body surface to ground. There is a level of current at which an individual cannot voluntarily release from the circuit. Thi
32、s is the “no let go current,” from which burns and death by electrocution can result. Studies have shown that perception of electrical shock begins when the current through affected body parts is about 0.001 amperes (1 mA). When the current increases to about 6-25 mA for women and 9-30 mA for men, m
33、uscle control is lost, and it becomes impossible to let go of the circuit. At NEMA ABP 9-2015 Page 2 2015 National Electrical Manufacturers Association higher values of current, e.g., above about 50-150 mA, extreme pain, respiratory arrest, and severe muscular contractions occur. This can lead to ve
34、ntricular fibrillation and/or heart stoppage, which causes certain death. The value of current depends on the bodys electrical resistance and the voltage applied. Ohms law (I = V/R), illustrates that an increase in current through the body occurs when either the applied voltage increases or the body
35、s resistance decreases. Electrical circuits of 120V can be just as lethal as 240V, 480V, 600V, or higher voltage circuits because the current through the body depends on the bodys resistance. Electrical shock can also result in, if not electrocution, other physical injuries. If a person is startled
36、or surprised by even a slight electrical sensation, an involuntary muscular reaction can cause them to pull away suddenly. A slip or fall can result in cuts, abrasions, or worse. II. Why Isnt the Power Turned Off? Lack of Proper Training. Many people are just not aware of the inherent dangers descri
37、bed above. Victims and witnesses of electrical accidents are often surprised at the violent and explosive nature of electrical energy: the fireballs, bright flashes, acrid smoke, and hot molten metal. In workplaces where a robust electrical safety program has not been implemented, safety training of
38、 electricians might be done informally and might and by instructors who have already developed bad habits. Sometimes, unqualified and unlicensed individuals work on electrical circuits, and safety training is given only “lip service”or there is no training at all. It is essential that safety trainin
39、g be emphasized to preclude any such complacency. Courses in electrical safety are provided by state and community colleges, by the electrical inspectors and training organizations for electricians, and by various contractors associations. Industry management can promote increased safety by requirin
40、g more employees receive formal safety training. To comply with NFPA 70E requirements, Employers are required to have documentation showing that qualified persons have been trained to safely perform their jobs. The Electrical Service “Cant” Be Interrupted. Countless electrical accidents have resulte
41、d from this philosophy. Invariably, the accidents cause major shutdowns, outages, and equipment replacement. And in the end, what “could not be shut down” is shut down! With detailed planning, almost any piece of electrical equipment can be taken out of service. While this planning might take additi
42、onal time and involve additional cost, the risk of not doing it might be an accident that results in massive equipment damage, personal injury, or death. The time and cost of an accident far exceeds the time and cost of a properly planned outage. The Job Must Be Done Quickly. When the pressures of t
43、ime dominate any work activity, mistakes and accidents invariably happen, and caution and good judgment give way to haste. Again, an accident would inevitably take more time to resolve. “Ive Never Had a Problem Before.” There is a common misconception that if a known safety practice is violated seve
44、ral times without resulting in an accident, then a future violation will not cause an accident either. Many electricians who receive safety training learn on 120V/240V circuits. Much of their work deals with 120V to ground. While it is possible to be shocked, burned, and/or electrocuted on 120V/240V
45、 systems, these individuals can lose their fear by continually working on energized equipment until it becomes “second nature.” A few shocks, sparks, and burned wires might not deter them, or it might be faster to make connections without having to turn off the power. Transferring 120V experience to
46、 480V and above can be a fatal error. The Equipment Needs to Be Energized to Perform Tests. It is recognized that there are some situations where electrical measurements must be taken while the equipment is energized. In these situations, there are legal requirements that must be met before any work
47、 is performed. The NEC (NFPA 70) is adopted as law by most states and many other government entities. This Code mandates that only “qualified persons” performing electrical work be permitted access to live parts.1 A qualified person is one who is “trained and knowledgeable of the construction and op
48、eration of equipment or a specific work method and . . . trained to recognize and avoid the hazards that might NEMA ABP 9-2015 Page 3 2015 National Electrical Manufacturers Association be present with respect to that equipment or work method.” The Occupational Safety and Health Act (OSHA) is a feder
49、al law requiring that employers provide a safe place to work. Beginning in January 2015, OSHA required that employers estimate the incident heat energy of any electric-arc hazard to which a worker might be exposed. Further, employers generally must provide workers exposed to hazards from electric arcs with protective clothing and other protective equipment with an arc rating greater than or equal to the estimated heat energy. In the OSHA Regulations2, reference is made to NFPA 70E, Standard for Electrical Safety in the Workplace. Both OSHA and NFPA
